Embedded c learning notes

1. 2 Base number 、8 Base number 、16 The conversion of base systems

Binary representation ：0B
Hexadecimal representation ：0X

Hexadecimal said

2. Base conversion method

For binary to hexadecimal, a hexadecimal equals four bits of binary , adopt 8421 Code calculation

3. C Language structure

meaning

The ability to assemble multiple different data into one type （ It can contain multiple different data types ）

Structure structure

Other uses of structs

Can pass typedef To define the structure , And define other parameters through this structure

4. excent usage

extern yes C A key word in a language , Usually used before variable name or function name , The function is to explain “ This variable / Functions are defined elsewhere , To quote... Here ”,extern Most of this keyword should be in the storage type of variables

External variables （ Global variables ）extern---- Global static storage
Standard definition format ：extern Type name Variable name ;

effect

Use this variable in advance , Or reference variables in other files
Reference resources ：extern（ External variables ）

5. const Usage method

effect

You can define an immutable variable , An error will be reported when there are other assignment operations （ Generally, this variable is capitalized to distinguish ）

const int MaxNum = 100;  // Maximum class size 

const And a pointer

const int *p1;
int const *p2;
int * const p3;

In the last case , The pointer is read-only , That is to say p3 The value of itself cannot be modified ; In the first two cases , The data pointed to by the pointer is read-only , That is to say p1、p2 The value of itself can be modified （ Point to different data ）, But the data they point to cannot be modified .

const And function parameters

stay C In language , Individually define const Variables have no obvious advantage , Totally usable #define Command instead of .const Usually used in function parameters , If the parameter is a pointer , To prevent modification of the data pointed to by the pointer inside the function , You can use it const To limit .

const He Fei const Type conversion

When a pointer variable str1 By const When the limit , And it's like const char *str1 This form , Indicates that the data pointed to by the pointer cannot be modified ; If you will str1 Assign a value to another one that is not const Decorated pointer variable str2, There's a risk . Because by str1 Can't modify data , And after assignment, it passes str2 Can modify the data , The meaning has changed , So the compiler doesn't advocate this behavior , Will give errors or warnings .

in other words ,const char * and char * It's a different type , Can't be const char * The data of type is assigned to char * Variable of type . But the reverse is possible , The compiler allows you to char * The data of type is assigned to const char * Variable of type .

This limitation is easy to understand ,char * The data pointed to has read and write permissions , and const char * The data pointed to has only read permission , Lowering data permissions will not cause any problems , However, increasing the authority of data may lead to danger .

C The parameters of many functions in the language standard library are const Limit , But we didn't pay attention to this problem in the previous coding process , Often non const Data of type is passed to const Type parameters , This has never caused any side effects , The reason is the above mentioned , Will not const Type conversion to const Type is allowed .

Reference resources C Language const A detailed explanation of the use of ,C Detailed definition of language constants

6. The pointer （* usage ）

1. Defining a pointer requires *（ as follows , Defines the pointer that should be , be called a）
2. When taking values, you need to use *（ as follows , Need to use * To represent the address of the data ）
3. For a defined pointer , no need * Represents the position of the pointer , Not the data

Reference to pointer variable

    printf("a=%d\n",a); // By name , Direct access to variables a Space （ Read )
    printf("a=%d\n",*p); // Through the address , Indirect access to variables a Space （ Read ）
    *p=6;// Equivalent to a=6; Indirect access to a Corresponding space （ save ）
    printf("a=%d\n",*p);

Wild pointer （ Misquote ）

int *pi,a; //pi uninitialized , No legal direction , by “ wild ” The pointer 
*pi=3; // Runtime error ！ Not right ” wild ” The space pointed to by the pointer is used for saving . This statement attempts to put  3  Deposit in “ wild ” The pointer pi In the random space referred to , Run time errors will occur .
a=*pi; // Runtime error ！ Not right ” wild ” The space fetch operation pointed to by the pointer . This statement attempts to start from “ wild ” The pointer pi Fetch data from the indicated space , Then assign it to the variable a It will also generate runtime errors .
/* Quote... Correctly */
// & To get a The address of 
pi=&a;// Give Way pi There is a legal point ,pi Point to a The space corresponding to the variable 
*pi=3;// hold 3 Indirect deposit pi The variable pointed to a The corresponding space 

The difference between pointer and multiplication

The following differences ：a multiply b,1 multiply b,a Add the pointer b Value ,a Multiply by the pointer b Value

7. Structure pointer

Just like ordinary variables , The structure pointer is the position of the represented structure , as follows ：

# include <stdio.h>
# include <string.h>
struct AGE
{
    
    int year;
    int month;
    int day;
};
struct STUDENT
{
    
    char name[20];  // full name 
    int num;  // Student number 
    struct AGE birthday;  // Birthday 
    float score;  // fraction 
};
int main(void)
{
    
    struct STUDENT student1; /* use struct STUDENT Structure type defines structure variables student1*/
    struct STUDENT *p = NULL;  /* Define a point struct STUDENT Pointer variables of struct type p*/
    p = &student1;  /*p Point to the structure variable student1 The first address ,  That is, the address of the first member */
    strcpy((*p).name, " Xiao Ming ");  //(*p).name Equivalent to student1.name
    (*p).birthday.year = 1989;
    (*p).birthday.month = 3;
    (*p).birthday.day = 29;
    (*p).num = 1207041;
    (*p).score = 100;
    printf("name : %s\n", (*p).name);  //(*p).name Can not write p
    printf("birthday : %d-%d-%d\n", (*p).birthday.year, (*p).birthday.month, (*p).birthday.day);
    printf("num : %d\n", (*p).num);
    printf("score : %.1f\n", (*p).score);
    return 0;
}

It can also be useful -> To point to a special variable of a pointer , as follows ：

# include <stdio.h>
# include <string.h>
struct AGE
{
    
    int year;
    int month;
    int day;
};
struct STUDENT
{
    
    char name[20];  // full name 
    int num;  // Student number 
    struct AGE birthday;  /* use struct AGE Structure type defines structure variables birthday,  Birthday */
    float score;  // fraction 
};
int main(void)
{
    
    struct STUDENT student1; /* use struct STUDENT Structure type defines structure variables student1*/
    struct STUDENT *p = NULL;  /* Definition struct STUDENT Pointer variables of struct type p*/
    p = &student1;  /*p Point to the structure variable student1 The first address ,  That is, the address of the first item */
    strcpy(p->name, " Xiao Ming ");
    p->birthday.year = 1989;
    p->birthday.month = 3;
    p->birthday.day = 29;
    p->num = 1207041;
    p->score = 100;
    printf("name : %s\n", p->name);  //p->name Can not write p
    printf("birthday : %d-%d-%d\n", p->birthday.year, p->birthday.month, p->birthday.day);
    printf("num : %d\n", p->num);
    printf("score : %.1f\n", p->score);
    return 0;
}

Reference resources ： Structure pointer description

8. Macro definition

Macro definition is a common preprocessing instruction , That is to use “ identifier ” To express “ Replace list ” The content in . The identifier is called the macro name , In the process of pretreatment , The preprocessor takes all the macro names in the source program , Replace with the contents of the replacement list in the macro definition .

There are parameter macro definitions

The format is ：

#define  identifier   Replace list 

The substitution list can be a numeric constant 、 character constants 、 String constants, etc , Therefore, a macro definition can be understood as using an identifier to represent a constant , Or symbolic constant .

Macro definition with parameters

The definition format of substitute meal macro is ：

#define  identifier ( Parameters 1, Parameters 2,..., Parameters n)  Replace list 

Reference resources Macro definition

9.Typedef

1. Define a new type name for the basic data type
   All basic types by default can be used typedef Keyword to redefine the type name

2. For custom data types （ Structure 、 Common body and enumeration type ） Define concise type names
   

3. Define concise type names for arrays

typedef int INT_ARRAY_100[100];
INT_ARRAY_100 arr;

4. Define concise names for pointers

typedef char* PCHAR;
PCHAR pa;

Reference resources typedef

10. Conditional compilation

Conditional compilation means that the preprocessor compiles instructions according to conditions , Conditionally select part of the source code as the output , Send it to the compiler to compile . It's mainly for selective operation , Prevent macros from replacing content （ Such as documents, etc ） The repetition of contains . The common conditional compilation instructions are shown in the table .

Conditional compilation instructions	explain
#if	If the condition is true , Then perform the corresponding operation
#elif	If the previous condition is false , And the condition is true , Then perform the corresponding operation
#else	If the previous conditions are all false , Then perform the corresponding operation
#endif	End the corresponding conditional compilation instructions
#ifdef	If the macro has been defined , Then perform the corresponding operation
#ifndef	If the macro is not defined , Then perform the corresponding operation

11. Memory operations

One 、malloc/calloc
malloc and calloc Can allocate memory areas , but malloc Only one memory area can be requested at a time ,calloc You can apply for multiple memory areas at a time ． in addition calloc The allocated memory area will be initially converted into ０,malloc No initialization ．
Two 、 malloc/calloc
free Can be released by malloc or calloc Memory allocated by the memory allocation function ． When the program is large , You may have to dynamically allocate memory multiple times during , If not released in time , The program will take up a lot of memory ．
3、 ... and 、 memset
memset hold buffer Before the memory area referred to count Bytes set to a character ASCLL value ． Generally used to give an array , String and other types of assignment ．
Four 、 memcpy
memcpy Will be able to src The memory area referred to is copied count Byte to dest The memory area referred to ． If count Than src The number of bytes is large ,strcpy Can copy ’/0’ After that ． it is to be noted that dest and src Don't overlap ．
memcpy Just copy memory space , Do not deal with spatial overlap ．